The present invention relates to tunable semiconductor lasers and particularly to single mode lasers for use in wavelength division multiplexed systems.
Single mode wavelength tunable semiconductor lasers are important components in wavelength division multiplexed (WDM) systems. In such applications, the essential parameter that determines the effectiveness of a tunable laser is the inter-channel switching time and the simplicity of the emission-wavelength controlling circuit. Currently, many fixed wavelength laser sources, such as distributed feedback (DFB) lasers, rely on temperature tuning to switch the wavelength between the various channels. In this context, the temperature of the laser is changed to create variations in the bandgap energy and the refractive index, leading to a change in the emission wavelength. The process is relatively simple, but is inherently too slow for fast switching applications.
An alternative approach to wavelength control is through electronic tuning, a technique which can be applied to a two-section distributed Bragg reflector (DBR) laser having a gain section and a Bragg section for tuning. The tuning of the output wavelength can be achieved by varying the wavelength xcexB of the Bragg section which satisfies the Bragg condition for constructive interference:
xcexB=2neffxcex9g
in which neff and xcex9g are the effective refractive index and the grating period of the Bragg section, respectively. In this approach, electrical current is injected into the Bragg section of the laser such that it is perpendicular to the surface of the device. This injection of current causes a change in the carrier density and a corresponding change in the effective refractive index, thereby altering the lasing wavelength that satisfies the Bragg condition. This technique of electronic wavelength tuning allows faster channel switching, such that the switching time is limited only by the speed of the integrated driving and controlling circuitry.
However, two-section DBR lasers suffer from discontinuous wavelength tuning, due to mode jumping from one longitudinal mode of the laser cavity to the next. Typically, a jump occurs every 2-3 nm over a 15 nm tuning range. This so-called mode-hopping occurs primarily because of interference and competition between the optical mode that is determined by the Bragg section and other residual longitudinal Fabry-Perot modes arising within the laser device. For a simple two section device comprising gain and Bragg sections, longitudinal Fabry-Perot modes can arise from cavities formed by Fresnel reflections at the air-gain interface, gain-Bragg region interface and air-Bragg region interface.
In order to reduce optical loss in the Bragg section of the DBR laser, so that higher output power can be achieved, it is desirable that the bandgap energy of the Bragg section be larger than that of the gain section. This is usually accomplished by a growth and regrowth technique resulting in different material composition in the waveguide layers of the gain and Bragg sections. However, this introduces a large mismatch in the transverse refractive index profile of the two sections, leading to the unwanted reflection at the gain-Bragg section interface. For example, a 100 nm detuning in the band edge of the Bragg section relative to the gain section, for a device based on an epitaxial layer of InGaAsP, would result in an abrupt refractive index difference of 0.08 leading to a gain-Bragg interface reflectivity of approximately 2xc3x9710xe2x88x924, if the waveguides in the two sections are perfectly aligned. However, such perfect alignment is almost impossible to realize given the typical nonuniformity across an epitaxially grown wafer. Thus, if there is also a vertical offset between the waveguides in the two sections, the interface reflectivity increases to 5xc3x9710xe2x88x924. Variation of the waveguide thickness across the interface will lead to an even higher value.
Even if the exposed facets of the laser device are coated with anti-reflection optical coatings, it is extremely difficult to completely eliminate all the reflections. In fact if, as is usually the case, the laser output is taken from the DBR end of the device, the facet at the opposing end is coated with a high reflectivity (HR) coating at the lasing wavelength, which can further exacerbate the problem of parasitic modes. The situation is even more severe for three, four or five section tunable laser diodes.
However, despite the increased number of Fresnel reflections, the conventional approach to solving this problem is to use a three-section DBR laser, comprising gain, phase and Bragg sections as described by S. Murata, I. Mito and K. Kobayashi in xe2x80x9cTuning ranges for 1.5 xcexcm wavelength tunable DBR lasersxe2x80x9d, Electron. Lett., vol. 24, pp. 577-579, 1988. In the three section laser, electrical current is also injected into the phase section to adjust the phase of the feedback from the DBR by means of carrier-induced refractive index changes. With the addition of a wavelength reference, this permits stabilization and fine-tuning of the mode frequency, resulting in continuous tuning of the device output wavelength over the entire available range. Other structures derived from the DBR laser are also used, including: sampled grating DBR (SG-DBR) described in C. K. Gardiner, R. G. S. Plumb, P. J. Williams and T. J. Ried, xe2x80x9cWavelength tuning in three section sampled grating DBR lasersxe2x80x9d, Electron. Lett., vol. 31, pp. 1258-1260, 1995; superstructure grating DBR (SSG-DBR) described in Y. Tohmori, Y. Yoshikuni, T. Tamamura, H. Ishii, Y. Kondo and M. Yamamoto, xe2x80x9cBroad-range wavelength tuning in DBR lasers with super structure grating (SSG), IEEE Photon. Technol. Lett., vol. 5, pp. 126-129, 1993; and grating coupled sampled reflectors (GCSR) described in M. Oberg, S. Nilsson, K. Streubel, J. Wallin, L. Backbom and T. Klinga, xe2x80x9c74 nm wavelength tuning range of an InGaAsP/InP vertical grating assisted codirectional coupler laser with rear sampled grating reflectorxe2x80x9d, IEEE Photon. Technol. Lett., vol. 5, pp. 735-738, 1993. All these devices are capable of continuous tuning with a larger wavelength tuning range.
However, these types of laser suffer from increased complexity of driving circuitry as compared to the two-section DBR laser. More electrodes (at least three) are required and matching of numerous input currents to the appropriate electrodes is necessary to accurately select a wavelength. In addition, as the device ages under conditions of constant electrical current bias, the carrier density in the semiconducting material will tend to decrease, owing to nucleation of non-radiative defects or increased leakage around the active layer. This leads to a drift of the emission wavelength which, in time, can become significant and may even lead to mode hopping. The use of multiple electrodes in tunable lasers such as the SSG-DBR requires more complex locking algorithms, with associated look-up tables, and the whole issue of wavelength stabilization becomes more complicated. Calibration of the emission wavelength for multiple-electrode tunable laser diodes is also extremely time consuming. Furthermore, the output power that can obtained from SG-DBR, SSG-DBR and GCSR lasers is comparably small, typically limited to a maximum of 10 mW.
Recently, continuous wavelength tuning has been realized by using a two-section DBR laser, disclosed in H. Debrxc3xa9geas-Sillard, A. Vuong, F. Delorme, J. David, V. Allard, A. Bodxc3xa9rxc3xa9, O. LeGouezigou, F. Gaborit, J. Rofte, M. Goix, V. Voiriot and J. Jacquet, xe2x80x9cDBR module with 20-mW constant coupled output power over 16 nm (40xc3x9750GHz spaced channels)xe2x80x9d, IEEE Photon. Technol. Lett., vol. 13, pp. 4-6, 2001. The operation of the laser in a continuous wavelength tuning mode is achieved through a combination of adjusting the gain current injection, Bragg voltage and temperature of the laser. Although the process is relatively simple, mode hopping occurs when the device is operated at a fixed temperature and control of at least three parameters, including the temperature of the laser, is required to achieve continuous tuning of the wavelength.
Therefore, there is a requirement for a simple technology by means of which hop-free, continuous wavelength tuning of a single-mode laser diode can be achieved, which provides for stable and fast wavelength switching with a minimal number of control parameters, thereby avoiding the high cost, complicated design and other problems associated with the present conventional approaches.
According to the present invention, a semiconductor laser comprises a gain section and an adjacent Bragg section, wherein output laser light is emitted via a facet at an interface between air and the gain section, the Bragg section comprising a distributed reflecting structure having a length substantially greater than required to ensure single longitudinal mode operation of the laser in which the side-mode suppression ratio (SMSR) is 35 dB or more, thereby in use substantially suppressing optical feedback from a facet at an interface between the Bragg section and air, and wherein an interface between the Bragg section and the gain section is quantum well intermixed, thereby rendering the interface substantially anti-reflecting at the wavelength of the laser.
Preferably, said facet is coated such that its reflectivity is low enough to substantially suppress unwanted Fabry Perot modes, but high enough for efficient laser operation. Typically, the reflectivity of the coated facet will be in the range 1-5%.
To eliminate mode-hopping, it is essential that any unwanted longitudinal Fabry-Perot modes be prevented. This can achieved by substantially eliminating the effective Fresnel reflection at two of the three interfaces associated with the device, namely the air-Bragg section interface and the gain-Bragg section interface.
A reduction in the contribution due to Fresnel reflection at the air-Bragg section interface can be accomplished by antireflection (AR) coating the facet. However, a greater reduction can be achieved by incorporating a Bragg section with a length longer than that typically used or needed to ensure single-frequency lasing operation. Due to the increased reflectivity of the distributed structure, this has the combined effect of reducing the amount of light reaching the air-Bragg section interface from the active gain region and also reducing the amount of reflected light returning to the active gain region.
Therefore, it is preferred that the laser comprises a Bragg section with a length substantially longer than that typically used or required to ensure single-frequency (SLM) lasing operation. Preferably, the facet at the air-Bragg section interface is also AR coated to further reduce the level of unwanted reflected light. However, the use of the longer Bragg section relaxes the requirements on the quality of the AR coating or can indeed negate the requirement for an AR coating.
However, there is still unwanted reflection associated with the gain-Bragg section interface. To eliminate this reflection, there should be no discontinuity in the optical waveguide running through the two sections and the optical mode profile in the two sections should be well matched. This requires a similar transverse refractive index profile for the two sections of the waveguide.
The reduction and smoothing of the refractive index discontinuity typically associated with the gain-Bragg section interface, can be achieved by creating the desired larger bandgap energy in the Bragg region, as compared to the gain region, via a process of quantum well intermixing (QWI), rather than using growth and regrowth techniques. QWI is a post-growth technique that allows tuning of the bandgap in localized sections of the device heterostructure. A good review of QWI has been written by J. H. Marsh in xe2x80x9cQuantum well intermixingxe2x80x9d, Semicond. Sci. Technol., vol. 8, pp. 1136-1155, 1993. By application of a QWI process to the Bragg section, the increase in bandgap energy can be achieved with minimal change to the refractive index profile. In addition, the waveguides in the Bragg and gain sections are inherently in close alignment as no regrowth phase is required.
It is therefore preferred that the band gap of the Bragg section is increased by a QWI process, such that said section is substantially transparent at the lasing wavelength and that the transverse refractive index profile of the waveguide in said section is well matched to that in the gain section. Preferably, the QWI extends a short distance, a few wavelengths, into the gain region so as to smooth any residual refractive index discontinuities.
The frequency tuning and stabilization of the aforementioned two-section DBR laser can be achieved by control of the electrical currents to the gain and Bragg sections of the device. By using appropriate control and feedback circuitry, the emission wavelength can be tuned continuously over a substantial portion of the available range, without mode hopping, while maintaining constant output power and large side-mode suppression ratio (SMSR). Compensation for wavelength drift due to device ageing is also simplified considerably.